In extruder-type continuous casting, concrete is extruded through the mold or nozzles of a moving casting machine by means of auger feeders and the ready-cast product remains setting on a stationary casting bed. The casting machine is propelled, e.g. by the reaction forces of the auger feeders. Other possible slip casting techniques are, e.g., the so-called slip-former technique. If so desired, hollow-core cavities can be made in the product during casting by means of shaping mandrels. The hollow-core cavities may be used, e.g., as installation ducts for piping and cables.
To accomplish such installations, an opening leading into the cavity has to be made in the surface of the hollow-core slab. The opening is usually made at the plant onto the slab resting on the casting bed by removing concrete at the cavity while the cast concrete still is fresh. If an opening is needed only in the bottom surface of the slab, it is necessary to make first an opening in the top surface of the slab resting on the casting bed only after which an opening in the bottom surface can be made. The opening made in the top surface of the slab is thus unnecessary and remains to be filled later, for instance at the construction site. Openings are also necessary in solid-core slabs, e.g. floor planks, e.g. for leading-through of sewer, air-conditioning and water pipes and electrical wiring. This kind of openings are also made at the plant by digging an opening from the top surface of the slab to the bottom surface of the slab.
Openings leading also into the hollow-core cavities are needed at the bottom surface of a hollow-core slab, e.g., as an outlet for water possibly accumulating in the cavities. These kinds of openings, e.g., relatively small water drainage openings, are normally made in the slabs at the plant the same way as described above and/or, e.g., at the construction site by drilling the finished product.
In slip casting, e.g. in the extruder or slip former techniques, the moving casting machine, instead of the casting bed, comprises also the sides that define the sides of the product. As soon as the continuously moving casting machine has traveled forward and the ready-cast product is left resting on the casting bed, the product must be in stable form and be self-supporting. This sets requirements for the concrete mix used, as known in the art. The fed concrete mix has to be dry enough so that after compaction during casting, also the hollow-core-cavities will retain their shape.
Earlier it has not been possible to incorporate separate cores on the casting bed in order to provide openings or recesses constituting provisions for openings in the bottom surface of the slab during slip casting using relatively dry concrete mix,